Xylo-oligosaccharide is a general term of oligosaccharides formed by β-glycosidic linkages of a plurality of xylose units. Xylo-oligosaccharides are also used as a material for functional foods because of, for example, its excellent intestine-regulating function (Aachary A. A. et al., Compre. Rev. Food. Sci. Food Saf. 10, 2-16 (2011)).
Xylo-oligosaccharides can be obtained through hydrolysis of xylan contained in cellulose-based biomass. Known hydrolysis methods include hydrothermal treatment method (Moniz P. et al., Ind. Crops. Prod. 62, 460-465 (2014)), acid hydrolysis method (Akpinar O. et al., Carbohydr. Res. 344, 660-666 (2009)), and enzyme treatment method (JP 4675139 B).
When biomass is to be hydrolyzed with enzymes, filamentous fungi are suitably applied because of their excellent capacity of producing cellulase which is an enzyme that degrades cellulose and xylan. However, it is known that cellulase produced by filamentous fungi degrades xylo-oligosaccharides into monosaccharide xylose in the reaction with biomass. β-xylosidase is known as an enzyme that degrades xylo-oligosaccharides into xylose, and β-xylosidase 1 derived from Trichoderma reesei has been reported to react with xylo-oligosaccharides from disaccharide (xylobiose) to heptasaccharide (xyloheptose) to generate xylose (Herrmann M. C. et al., Biochem. J. 321, 375-381 (1997)).
β-xylosidase 1 belongs to glycoside hydrolase family 3 (GH3) (Carbohydrate-Active EnZYmes Database). Enzymes belonging to GH3 comprise a plurality of highly conserved regions (domains), and β-xylosidase 1 of Trichoderma reesei comprises N- and C-terminal domains of GH3. In β-xylosidase of Trichoderma reesei, amino acid residues at positions 264, 311, and 464 are considered essential for its activity (Rasmussen L. E. et al., Biotech. Bioeng. 94, 5, 869-876 (2006) and Margolles-Clark E. et al., Appl. Environ. Microbiol. 62, 10, 3840-3846 (1996)). Other than these domains, β-xylosidase 1 comprises Fn3-like domain, but the function of Fn3-like domain is not known.
Thus, there is a problem with the use of cellulase of a fungus belonging to the genus Trichoderma in that it degrades xylan into xylose.
The Applicant hereby incorporates by reference the sequence listing contained in the ASCII text file titled SequenceListing.txt, created Sep. 26, 2018 and having 35.9 KB of data.